A comparative study for the removal of methylene blue dye by N and S modified TiO 2 adsorbents Shila Jafari a, ⁎, Feiping Zhao a,b , Dongbo Zhao b , Manu Lahtinen c , Amit Bhatnagar d , Mika Sillanpää a a Lappeenranta University of Technology, LUT Chemistry, Laboratory of Green Chemistry, Sammonkatu 12, FI-50130 Mikkeli, Finland b Colleges of Chemistry & Chemical Engineering, Hunan Normal University, China c University of Jyväskylä, Department of Chemistry, Laboratories of Inorganic and Analytical Chemistry, P.O. Box 35, FI-40014 Jyväskylä, Finland d Department of Environmental Science, University of Eastern Finland, P.O. Box 1627, FI-70211 Kuopio, Finland abstract article info Article history: Received 9 November 2014 Received in revised form 26 January 2015 Accepted 14 March 2015 Available online 17 March 2015 Keywords: Methylene blue TiO 2 Adsorbent Kinetics Isotherms pH Successful removal of methylene blue (MB) dye from aqueous solutions using nitrogen and sulfur modified TiO 2 (P25) nanoparticles has been demonstrated in this study. The modified adsorbents were characterized using various analytical methods, such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The adsorption potential of S-TiO 2 , N-TiO 2 and TiO 2 (P25) type ad- sorbents was tested for the removal of MB dye. The kinetic studies indicated that the adsorption of MB dye followed the pseudo-first order model, while desorption processes followed the second order model. The adsorp- tion capacity of the adsorbent proved to be increasing as a function of initial pH of the solution. The maximum adsorption capacities were found to be 350.66, 410.12 and 282.84 mg/g of S-TiO 2 , N-TiO 2 and TiO 2 (P25), respec- tively. It can be concluded that modification of TiO 2 (P25) nanoparticles with N and S leads to a higher adsorptive removal of MB. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Dyes are used as coloring substances in plethora of industries, such as various textile industry applications, food, paper, carpets, rubbers, plastics and cosmetics [1–3]. The discharge of colored wastewater from these industrial plants into natural streams has caused many sig- nificant problems such as increasing the toxicity and chemical oxygen demand (COD) of the effluent, and also reducing light penetration, which has a derogatory effect on photosynthetic phenomena. From the aesthetic and health point of view, the presence of dyes (carcino- genic compounds in particular) in surface and underground waters is neither not safe, pleasant, nor welcomed [4]. Methylene blue (MB), a cationic dye, is one of the dyes that is used extensively for dying cotton, wool and silk. It is currently estimated that 30–40% of the used dyes originating from industrial sources are released into waste waters [5]. These dyes are chemically and photolytically stable and the complex ar- omatic structures of these substances may hinder their natural biodeg- radation processes. Therefore, color removal from these waste waters has been attracted much attention [6]. Recently, research efforts have focused on the processes dealing with removal of different types of dyes from wastewaters by physical and chemical methods. These methods include ozonation, membrane separation, electrochemical treatment, ultrasonic techniques, photo- catalysis and adsorption [7–9]. All these processes have their merits and disadvantages and among them, the adsorption process is preferred as an environmentally friendly and cost effective technique [5]. Addi- tionally, a literature survey shows that the selection of adsorbent plays an important role in determining its economic feasibility [10–13]. For this purpose, the search for efficient and low cost adsorbents is ongoing. Although there are a wide variety of adsorbents, the majority of studies focus on the most common adsorbents, such as chitosan, acti- vated carbon, fly ash and sepiolite [5,14–21]. Despite the availability and low-cost of titanium dioxide (TiO 2 ), its use has largely been overlooked as dye adsorbent [22–24]. TiO 2 is widely used as a photocatalyst and as an ideal adsorbent for the degradation of various organic pollutants. TiO 2 is a very promising adsorbent due to the high surface reactivity, ad- sorption capacity and low-cost. Furthermore, as the pH of zero point charge (pH pzc ) of TiO 2 is ca. 6.0–6.8 [25,26]. It is a suitable adsorbent for the adsorption of charged groups due to its favorable electrostatic at- traction mechanism. Most of the studies concerning TiO 2 as an adsorbent, concentrated on adsorption capacities, mechanisms, process parameters to mention a few [9,27–29]. Moreover, the adsorption ability of modified TiO 2 was also investigated. Janus et al. reported that nitrogen modified TiO 2 showed higher adsorption capacity for the removal of Reactive Red 198 and Direct Green 99 as compared to an unmodified TiO 2 [30]. Li et al. [22,31] described dark adsorption and photodegradation experi- ments of Orange II dye on TiO 2 supported porous adsorbents at different Journal of Molecular Liquids 207 (2015) 90–98 ⁎ Corresponding author. E-mail addresses: Shila.jafari@lut.fi, shila.sanaz.jafari@gmail.com (S. Jafari). http://dx.doi.org/10.1016/j.molliq.2015.03.026 0167-7322/© 2015 Elsevier B.V. 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